Each histotropsy pulse generates a dynamically changing cluster of microbubbles. The bubbles behave as one dynamic unit, growing and shrinking together, and therefore this bubble cluster can be called a "bubble cloud" as defined by previous cavitation researchers.
Important time scale:
Histotripsy Pulse: several μsec
Collapse cycle of the bubble cloud: several hundred μsec
Residual bubbles from cloud collapse: several msec
Xu Z, Raghavan M, Hall TL, Chang C-W, Mycek M-A, Fowlkes JB, Cain CA, "High Speed Imaging of Bubble Cloud in Pulsed Cavitational Ultrasound Therapy - Histotripsy," Proc. IEEE International Ultrasonics Symposium, 1D-4 (2006).